Bind 'in' operator more tightly than boolean operators.

When I added support for the 'in' operator a couple months ago, I think I got the precedence slightly wrong.

JS has no 'in' operator, so there's nothing to base nunjucks' behavior off there, but Python (https://docs.python.org/2/reference/expressions.html#operator-precedence) and Jinja both bind 'in' more tightly than boolean operators, so that this `{% if msg.status in ['pending', 'confirmed'] and msg.body %}` does not evaluate as `{% if msg.status in (['pending', 'confirmed'] and msg.body) %}` (which is what nunjucks master currently does).

I think the Python/Jinja behavior is more useful and logical than nunjucks' current behavior: `in` is a comparison operator, like `==` et al, and should bind with similar precedence. You wouldn't expect `{% if msg.status == 'pending' and msg.body %}` to evaluate as `{% if msg.status == ('pending' and msg.body) %}`.

This pull request fixes the precedence (and while I was at it, also adds a test to confirm that `and` binds more tightly than `or`, which is correct and matches both Python and JS).